A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), a ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. To elucidate additional information regarding a sample, the MS system may be configured for carrying out tandem MS, or MS-MS, experiments. In this case, selected ions produced by the ion source, or “parent” ions, are dissociated into fragment ions (or “daughter” ions) in a collision cell. A mixture of the parent ions and fragment ions may then be transferred into the mass analyzer, and the resulting mass spectrum thus includes the fragment spectra.
Tandem MS may be implemented in a triple quadrupole (or QQQ) MS system, which includes three quadrupole devices in series. The first quadrupole is utilized for mass selection, the second quadrupole is an RF-only device enclosed in a gas chamber and utilized as the collision cell, and the third quadrupole is utilized as the mass analyzer. Tandem MS may also be implemented in a quadrupole time-of-flight (or qTOF) MS system, the main difference being that the mass analyzer is a TOF analyzer instead of a quadrupole device. In either system, the first quadrupole is operated as a mass filter and thus is capable of passing only a single parent ion at a time. All other ions are lost, eliminating the opportunity to use these ions to contribute to signal intensity. The sample introduced to the ion source may, however, yield hundreds to thousands of different parent ions (parent ions having differing m/z ratios), and each parent ion in turn may yield tens of different fragment ions in the collision cell. In the QQQ or qTOF system, obtaining fragment spectra from several parent ions requires repeating the ion selection, fragmentation and analysis sequence for each parent ion several times. The number of experimental repetitions needed or desired may not, however, be compatible with the time constraints imposed on the MS system. This is particularly the case when the MS system is employed to analyze sample components eluting from the column of a liquid chromatograph (LC) or gas chromatograph (GC). On the other hand, simply loading parent ions having a range of m/z ratios into a collision cell simultaneously is typically not an acceptable solution, as this approach typically does not enable the identification of which parent ion created which fragment ion.
Therefore, there is a need for MS-MS systems and methods that enable the collection and mass spectral analysis of all combinations of parent ions (or any desired subset of parent ions) and fragment ions from a sample of interest. There is also a need for MS-MS systems and methods capable of performing such ion collection and analysis within the time constraints imposed by any sample introduction process that may be done at the front end such as, for example, LC or GC elution.